Animated Solution for Chemistry - s and p-Block Elements: Which of the following statements is true ?

Imagine you are a proton ($H^{+}$) attached to a massive, complex molecule. Your desire to leave and swim freely in the solution depends entirely on how well your parent molecule can handle the breakup. If the molecule you leave behind (the conjugate base) is highly stable and comfortable carrying a negative charge, you are free to go! This makes the original molecule a strong acid.

In this problem, we are tasked with finding the true statement among four options by comparing the acidic strengths of various chemical species. Let's break down the chemistry behind each option and uncover the truth.

Before we dive into the options, we must establish the fundamental principle of acid-base chemistry: The strength of an acid is directly proportional to the stability of its conjugate base.

When an acid donates a proton, it leaves behind a pair of electrons, forming a negatively charged conjugate base. Nature hates localized, concentrated charge. If the conjugate base can spread out (delocalize) this negative charge through mechanisms like electronegativity or resonance, it becomes highly stable. A stable conjugate base means the forward reaction (losing the proton) is highly favored.

Options A, B, and C

Let's evaluate the first three statements to see why they fall short.

Option (a): $H_{3}P{O}_3$ vs. $H_{2}S{O}_3$ This statement claims that Phosphorous acid ($H_{3}P{O}_3$) is stronger than Sulfurous acid ($H_{2}S{O}_3$). To compare them, we look at the central atoms: Phosphorus (P) and Sulfur (S). Sulfur is positioned to the right of Phosphorus in the periodic table, making it significantly more electronegative. A more electronegative central atom pulls electron density away from the O-H bonds more effectively, weakening the bond and making it easier for the $H^{+}$ to detach. Therefore, $H_{2}S{O}_3$ is actually the stronger acid. Statement (a) is false.

Option (b): HF vs. HCl This is a classic trap! Fluorine is the most electronegative element on the periodic table, so one might intuitively think Hydrofluoric acid (HF) is the strongest hydrohalic acid. However, acidic strength in this group is dominated by bond dissociation energy, not just electronegativity. The Fluorine atom is incredibly small, resulting in a very short and exceptionally strong H-F bond. It holds onto the proton tightly. In contrast, the Chlorine atom is larger, making the H-Cl bond longer and weaker. Consequently, HCl releases its proton much more easily in an aqueous medium. Statement (b) is false.

Option (c): $HCl{O}_4$ vs. $HCl{O}_3$ Here, we compare Perchloric acid ($HCl{O}_4$) and Chloric acid ($HCl{O}_3$). When they lose a proton, they form the perchlorate ($Cl{O}_4^{-}$) and chlorate ($Cl{O}_3^{-}$) ions, respectively. The perchlorate ion has four oxygen atoms over which it can delocalize its negative charge, resulting in four equivalent resonance structures. The chlorate ion only has three. More resonance structures mean greater delocalization and higher stability. Thus, $HCl{O}_4$ is a stronger acid than $HCl{O}_3$. Statement (c) is false.

Nitric vs. Nitrous Acid

Finally, we arrive at option (d), which compares Nitric acid ($HN{O}_3$) and Nitrous acid ($HN{O}_2$). Let's apply our golden rule and examine their conjugate bases.

When $HN{O}_3$ loses a proton, it forms the nitrate ion ($N{O}_3^{-}$). In this ion, the central nitrogen atom is bonded to three oxygen atoms. The negative charge is not stuck on just one oxygen; it is beautifully delocalized across all three oxygen atoms through three equivalent resonance structures.

On the other hand, when $HN{O}_2$ loses a proton, it forms the nitrite ion ($N{O}_2^{-}$). This ion only has two oxygen atoms available to share the burden of the negative charge, resulting in only two equivalent resonance structures.

Because the nitrate ion has more extensive resonance delocalization, it is significantly more stable than the nitrite ion. A more stable conjugate base means the parent acid is stronger. Therefore, $HN{O}_3$ is indeed a stronger acid than $HN{O}_2$.

Statement (d) is the absolute truth, perfectly demonstrating the profound impact of resonance stabilization in chemical reactivity!